Growth factor involvement in the carcinogenesis cascade is centered on the promotionary events of clonal expansion. These proliferative factors first appear early on in tumor development at sites of premalignant lesions or carcinoma in situ. Thus, a rational approach for defining early markers and target sites of cancer intervention is achieved by identifying the specific growth factors of promotion. We have used this investigative strategy as the primary focus of our experimental studies. Two approaches have been utilized to accomplish our investigative goals which include 1) Growth of human tumor cell lines in serum- free/hormone-free medium (RO) and the characterization (molecular/biochemical) of the autocrine growth factors generated under these conditions and 2) Defining biologically relevant peptides with a potential for growth promoting activity based on the presence of alpha- amidated carboxy-terminal amino acids. We have determined, from our work with RO adapted human tumor cell lines, that the expression of transferrin (Tf) and the transferrin receptor (TfR) are critical components which allow the cells to grow under such stringent conditions. Further analysis of pathological specimens by immunohistochemistry and in situ PCR has demonstrated that Tf and TfR are also expressed in a variety of human tumors but absent in normal counterpart tissue. In addition, Tf/TfR expression appears to track with growth demands and has been identified in regions of tissue damage, atypia and hyperplasia. These results implicate Tf and TfR as potential early markers of carcinogenesis and define a rational biologic site for intervention in tumor development.